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The first edition of the Technical Specification (TS) on Primary Optics for Concentrator Photovoltaic (CPV) Systems, IEC TS 62989:2018, has been published by the International Electrotechnical Commission (IEC), on March 8th, 2018. TS 62989 covers aspects related to the primary optics including: product identification, optical characteristics, mechanical characteristics, materials, (design) geometry, and visual appearance. This paper focuses on the key aspects of the norm, including optical performance, in order to promote the awareness and use of the standard. We describe the three methods intended to measure the key optical characteristics of the primary elements, i.e. focal spot size and optical efficiency, by using the encircled energy. The three methods are using different light sources, optical components and receiver sensors, but yielded very similar results in a round robin test. This justifies the continued use of all three methods. The use of a website (opticstests.pbworks.com) for documentation of discussions and references was novel to the standard development. The website proved to be useful for the introduction of new members of the group. The website helped to keep track of changes to the document as well as the required actions of the project team.The first edition of the Technical Specification (TS) on Primary Optics for Concentrator Photovoltaic (CPV) Systems, IEC TS 62989:2018, has been published by the International Electrotechnical Commission (IEC), on March 8th, 2018. TS 62989 covers aspects related to the primary optics including: product identification, optical characteristics, mechanical characteristics, materials, (design) geometry, and visual appearance. This paper focuses on the key aspects of the norm, including optical performance, in order to promote the awareness and use of the standard. We describe the three methods intended to measure the key optical characteristics of the primary elements, i.e. focal spot size and optical efficiency, by using the encircled energy. The three methods are using different light sources, optical components and receiver sensors, but yielded very similar results in a round robin test. This justifies the continued use of all three methods. The use of a website (opticstests.pbworks.com) for documentation ...

Ultra-precision molded polymer optics range from high precision imaging objectives to tiny lenses like those used in camera modules for cell phones, where centration tolerances and filling of small features is a challenge. We propose a manufacturing process termed Compression Molded Polymer Aspheres (COMPAS). Polymer preforms are inserted into mold cavities, and isothermally heated above glass point. Novel tooling has been developed to produce high volumes of COMPAS optics at reasonable cost and cycle time, using large scale parallelization of mold cavities. First results of the COMPAS process are very encouraging: shape accuracy (

The ISPA (Imaging Silicon Pixel Array)-tube is a position sensitive photon detector based on hybrid technology. It detects light via a photocathode and an appropriate electric field accelerates the emitted photoelectrons towards a silicon pixel anode. This anode, finely segmented into pixel detectors provides binary images and allows for the self-triggering of the tube. Coupled to scintillating crystals, ISPA-tubes have been successfully tested in the field of gamma ray imaging for biomedical applications, demonstrating real capabilities in both space and energy resolution. Recently, we have demonstrated the possibility to use YAP:Ce scintillating crystals as active windows for hybrid photon detector tubes, the photocathode being directly evaporated at their inner surface. We present results obtained with a new ISPA-prototype equipped with such a window and discuss the benefit of having an input window acting as the gamma-detecting medium, in both energy and space resolution points of view.

Hybrid Photon Detectors (HPD) detect light via photocathodes and accelerate the emitted photoelectrons by an electric field towards silicon PIN-anodes, where they are absorbed and generate electronic signals. We have developed two specific types of HPDs: (1) Hybrid photomultiplier tubes for photon counting and gamma spectroscopy; (2) Imaging silicon pixel array tubes for optoelectronic cameras. This paper will illustrate the main achievements, which we obtained in the last years, and will describe and discuss our present main RD 29.40; 29.30; 87.59

Pure crystalline YAlO3 can be applied due to its high refractive index as a novel material for optical elements within 240–1000 nm light transmission. If used as front-windows for photon detectors and coated with a photocathode at its vacuum interface, it improves their light collection efficiency. Its high refractive index increases the numerical aperture of lenses and hence their spatial resolving power. It matches favorably with the refractive index of MgF2, which used as antireflection coating reduces the surface reflectance close to zero. Doped with Ce3+-ions, YAlO3(Ce) can be readily used as scintillating front windows of vacuum-sealed photon detectors for ionizing radiation.

A YAP(YAlO/sub 3/:Ce)-scintillating window, coated on its inner surface with an S20-photocathode, seals a cross-focusing hybrid photomultiplier tube (HPMT) equipped with a small p-i-n anode of 2-mm diameter. This new radiation detector separates X-ray lines down to about 2-keV peak energy from the HPMT noise. Its detection efficiency for high gamma energies depends on the YAP-window thickness and amounts to about 18% attenuation at 400-keV energy in the present version. Competitive radiation detectors like Si photodiodes and Si drift chambers are discussed and compared to our prototype, with particular attention given to their energy resolution and noise performance, which limits their active area considerably. (19 refs).

The yearly energy collection efficiency of stationary solar concentrators can be evaluated using reverse ray-tracing, and a solar radiation model. In reverse ray-tracing, rays originating at the receiver of the concentrator are traced towards the surrounding hemisphere. The method allows for the evaluation of the absolute energy collection: new concentrators may be optimized for location and tilt, requiring one-time ray-tracing. The tilt of existing concentrators is optimized. Only possible solar incidence is considered by our model. The method is fast and realistic; it can be modified for concentrators in tilt operation.

One of the main problems in using parabolic concentrators with standard photovoltaics (PV) cells is the highly non-uniform illumination of the cells. The non-uniform irradiation causes high resistive losses in the standard cells due to their relatively high series resistance. This results in a considerably lowered efficiency. To solve the problem, we introduce three different structured reflectors that will create a more uniform illumination, and also increase the concentration ratio in certain cases. The structures were evaluated in an existing trough system by Monte Carlo ray tracing, and it was found that structures improve the system performance mainly by homogenizing the light on the cells. The yearly irradiation collected in the evaluation system is slightly lower than for a reference with smooth reflectors, but the more uniform illumination of the cells will generate a net increase of the total system performance compared to a system that was optimized with smooth reflectors. The benefit of the increased concentration ratio is increased flexibility in designing new systems with concentration ratios surpassing the limit of existing trough concentrators.

The Imaging Silicon Pixel Array (ISPA)-tube is a position-sensitive hybrid photon detector. Originally developed for high-energy physics purposes, it has also been used for biomedical applications. Two kinds of ISPA-tube prototypes have been tested successfully in the field of gamma ray imaging. The current developments aim at obtaining a detector dedicated to single-photon emission imaging. In this paper, we present the first use in a gamma camera of a new ISPA-tube prototype having an increased active input surface of 40 mm diameter and a de-magnifying electron optics. The quartz input window of the tube is optically coupled to a 3.5 cm 2 YAlO 3 :Ce detector array with 0.6 mm 2 single elements.

Shaped nonimaging Fresnel lenses have been designed according to the edge-ray principle, incorporating any combination of two acceptance half-angle pairs. A numerical solution yields nonimaging lenses consisting of minimum-deviation prisms. If the outer surface of the lens must be smooth, the lens shape will be convex. A linear lens prototype intended as concentrator for the collection of solar energy has been designed, manufactured and tested. Results in terms of flux distribution and the optical concentration ratio are presented. Shaped nonimaging Fresnel lenses are suited for application as solar concentrators, or as collimators, or in lighting applications, where they can fulfil technological requirements as well as being adaptable to the necessities of fashionable design.

An optimum convex shaped nonimaging Fresnel lens is designed following the edge ray principle. The lens is evaluated by tracing rays and calculating a projective optical concentration ratio. This Fresnel lens is intended for use in evacuated tube type solar concentrators, generating mid-temperature heat to drive sorption cycles, or provide industrial process heat. It can also be used along with a secondary concentrator in photovoltaic applications.

A 0.3-mm thin YAlO/sub 3/(Ce) scintillating crystal plate of 22 mm diameter was mounted inside an electrostatic-focussed hybrid photomultiplier tube (HPMT). The photocathode was evaporated directly on this scintillating plate opposite to the HPMT-anode. The HPMT was vacuum-sealed with a 1-mm thick Be radiation entrance window. Photoelectron numbers at their peak energies of photoabsorption and energy resolutions were measured between 3.69 and 59.6 keV. The average number of photoelectrons produced per keV radiation energy was determined to be 4.457 phel/keV and the energy resolution at 59.6 keV was 17%. Introducing in the same way such a thin YAlO/sub 3/(Ce) plate inside an imaging silicon pixel array (ISPA) tube would improve its spatial resolution, since it depends on the thickness of the X- ray detector.

A new class of photodetectors, the Hybrid Photo Detectors (HPD), is coming up as a major breakthrough in photodetection. The principle of the HPD is to couple a fully depleted silicon diode or silicon diode array to a photocathode in a vacuum tube. At present HPD's with one diode or a few diodes (Hybrid Photo Multiplier Tubes, HPMT) are commercially available (DEP, The Netherlands, and Hamamatsu, Japan) and a position sensitive photon detector (the Imaging Silicon Pixel Array tube, ISPA tube) is under development in our group (in collaboration with the RD-19 collaboration at CERN). We shall present the main features of HPD's and then our main results and applications for both HPMT's and ISPA tubes on photon counting, on imaging and on high energy physics applications.

The ISPA (Imaging Silicon Pixel Array)-tube is a position-sensitive photon detector. Originally developed for HEP applications, it can also be envisaged for biomedical applications. By coupling it to scintillating crystals or crystal arrays (YAP) we built a γcamera and detected through a collimator the 122 keV γ-rays from a 57 Co source. This paper presents the results obtained with two different ISPA-tube prototypes: the first ISPA-tube, having a glass fibre optic input window, allows to achieve very high spatial resolutions; the second ISPA-tube prototype, with its non fibre optic quartz input window, leads to much better energy resolutions while preserving sub-millimetrie spatial resolutions.

We have developed a new type of beta camera based on an imaging silicon pixel array (ISPA) tube combined with planar plastic scintillators or with SiY/sub 2/O/sub 5/:Ce scintillating powder. The ISPA tube consists of a photocathode viewed at 3 cm distance by a silicon anode divided into 1024 rectangular (75 /spl mu/m/spl times/500 /spl mu/m) detector pixels, each bump-bonded to its equally sized electronic pixel. Depending on the beta detector thickness we achieved spatial resolutions (FWHM) between 105-/spl mu/m (/sup 63/Ni source and 30-/spl mu/m-thick plastic scintillator) and 240-/spl mu/m (/sup 90/Sr-/sup 90/Y source and 120-/spl mu/m-thick plastic scintillator) by covering the detectors with brass templates. With their four 60-/spl mu/m-wide slits oriented parallel to the long pixel edges, we simulated small-sized beta strips. The impact of detector thickness is explained by multiple scattering, angular aperture of the template slits, and scintillating light distribution at the ISPA photocathode. Beta detection sensitivities were measured with calibrated /sup 3/H (tritium)- and /sup 14/C-Amersham microscale sources. They amount to 0.1 Bq (/sup 3/H) with 150 min counting time and to 0.025 Bq (/sup 14/C) with 180 min counting time.

We investigated the transmission of monochromatic light through porous media consisting of packings of glass beads (bead diameters d in the range of 0.16–0.4 mm) immersed in water. The light propagation through such strongly scattering random media is characterized by the transport mean free path l ∗ (distance over which the direction of propagation is randomized) and the diffusive absorption length L a . We determined these parameters by combining measurements of total transmission (in the strong absorption regime) with reference measurements on samples with known l ∗ . We obtain l ∗ ≈ 7d at the wavelength of 633 nm and l ∗ ≈ 9d at 515 nm; the absorption length L a lies between 5 and 10 mm.

The newly developed Imaging Silicon Pixel Array (ISPA) tube consists of a photocathode viewed at a 30-mm distance by a silicon chip, which contains 1024 pixels with 75 /spl mu/m/spl times/500 /spl mu/m edges. With this tube we imaged, as an example of a weak light source, /spl beta/-tracks (/sup 90/Sr) traversing a fused square bundle (2.5 mm edges), which contains 1600 individual scintillating fibers of 60-/spl mu/m transverse dimension. Simultaneously we counted the number of photoelectrons/mm (hit density) at different source positions along the 2-m fiber bundle, with potential differences varying from 10 kV to 26 kV between photocathode and pixel anode of the ISPA tube, and at different threshold settings of the pixel chip. The obtained hit densities are compared with those measured with a Hybrid Photomultiplier Tube (HPMT), which contains a silicon pin diode as anode instead of a pixel chip. >

We report the first observation of ultrasonic modulation of multiple light scattering speckles. The modulation at ƒ a = 2 MHz of the temporal field autocorrelation function 〈E(0)E ∗ (t)〉 of the light scattered from concentrated aqueous suspensions of polystytrene beads was measured. In addition, when using ƒ a = 27 MHz , the light intensity spectra measured with a Fabry-Perot interferometer show four inelastic peaks at ƒa and 2ƒa from the principal Rayleigh peak. The modulation amplitudes obtained from both techniques were found to increase with the ultrasonic amplitude and to vary with the mean free path of light in agreement with our simple model.

We measured the number of photoelectrons released from the photocathode of a newly available hybrid photomultiplier tube (HPMT) by photons from scintillating fibres. About 1600 individual 60 μm diameter fibres are contained in fused square bundles of 2.5 mm edges. The number of photoelectrons per mm fibre path changes from 10.6 at zero to 1 at 2 m bundle length. The potential difference between the HPMT photocathode and its electron detecting silicon diode amounted to 13 kV.

In comparison with a photomultiplier tube, the resolving power for photoelectron peaks is largely improved by a silicon diode bombarded with electrons from a light-detecting photocathode. With photoelectrons accelerated 10 13 keV, the measured variance of the electronic noise is 1.29 keV and the total variance of the photoelectron peaks is 1.81 keV. The continuum part of the photoelectron spectrum is explained by a backscattering process on the diode surface. The contrast function shows that up to 14 photoelectron peaks can be resolved.

The CPV community is still undecided on one critical issue: what material to use best for Fresnel lens parquets. Reliability and longevity are the most important, but all other properties play roles as well. We have developed and manufactured Fresnel lenses with the two commonly used materials: PMMA (Polymethylmethacrylate) and silicone on glass (SOG). Both lenses are designed for the same optical train for best comparability. This allows for better understanding the pros and cons of the materials and making an informed choice for a specific CPV module. While PMMA lenses are embossed from pre-fab sheets in a hot-cold process, the silicone lenses are cast from a heat-curing silicone rubber at moderate temperatures, reducing the energy consumption. PMMA allows for the inclusion of custom low-profile 3D (2.5D) structures for module assembly and mechanical alignment, a feature not possible in silicone due to its low rigidity. Both lenses suffer from thermal expansion and refractive index change. While PMMA parquets expand isotropically, SOG prisms deform due to the difference of expansion coefficients between the glass and the silicone. SOG lenses are prone to delamination of the silicone film. The adhesive strength of the film to the glass can be measured using a modified blister test that we developed. The results show large difference with different materials and confirm the necessity of controlling this issue closely. While the small thermal expansion of the glass sheets allows for larger parquet sizes, the deformation of the prisms with temperature may cause a performance hit.

Optics with high optical efficiency and reliability are the key components for CPV modules as well as high efficiency solar cells and a high accuracy tracker. The present paper describes the optical design, simulation and materials, including a direct comparison of geometrically identical lens designs for different materials i. e. PMMA (or acrilic) and silicone-on-glass (SOG) respectively, and glass secondary in three different geometries. The Fresnel lenses manufactured as 5×4 monolithic parquets are called Triple Primaries, and serve as test samples and off-the-shelf products of Concentrator Optics GmbH.

Fresnel lenses for CPV are made of PMMA or silicone-on-glass (SOG). We discuss these optical materials in terms of spectral transmissivity, refractive index, longevity, bankability and cost. Thermal expansion is most critical. Article not available.

We measured the attenuation losses and the corresponding number of photoelectrons in small diameter fibre bundles. Accordingly, two experimental setups have been developed for this purpose. The improvements in light yield of the last two years, due to better fibre production and to optimization of materials, concentrations and geometrical characteristics, are shown and discussed in detail. For 60 μm diameter fibre bundle, they amount to a factor 2 at the near end and to a factor 8 at 2 m distance from the light detecting photocathode.

Fused bundles of 60 μm diameter scintillating fibres with square cross sections of 2.5 mm edge length, were illuminated through 2 m length and their exit faces magnified and then photographed with a highly resolving CCD-camera. Subsequently, rows of individual fibres were selected with an appropriate slit or grid in front of the entrance face. The pictures taken, stored in a MacIntosh computer, were processed to determine the coherency, the intrinsic optical precision and the resolution of these bundles without distortions by optoelectronic readout systems: the precision (sigma) varied from 18.4 to 22 μm and the resolution from 42 to 55 μm, depending on the angular deviation from parallel slit orientation.

Replacement of a quartz photocathode window by an YAlO 3 (YAP) window yielded improved light transitions from BGO crystals (1.78 times) and PbWO 4 crystals (1.76 times) to the photocathode. This improvement is due to the higher refractive index of YAP (1.95), which matches much better the indices of BGO (2.14) and PbWO 4 (2.18) than quartz (1.47).

Secondary optics are used in concentrating photovoltaic (CPV) systems with Fresnel lens primaries to increase the optical system efficiency by catching refracted light that otherwise would miss the receiver, better the tracking tolerance (acceptance half-angle) and enhance the flux uniformity on the cell. Several refractive secondary optics under the same Fresnel lens primary are designed, analyzed and compared based on their optical performances, materials, manufacturability, manufacturing tolerancing and cost. The goal of this work is to show the basic two different design approaches statistical mixing as opposed to deterministic mixing. Caustics are elementary in the deterministic tailoring approach. We find that statistical mixing offers higher flexibility for the solar application. It is also shown that there are conventional, i.e. designs based on conic section ("half-egg") that work well as solar secondaries. It is also made clear that primary and secondary must be designed as optical train.

Environmental stress cracking (ESC) begins with crazes on the surface of the plastic. Plastic optics may corrode due to ESC. Usually ductile, plastic may become brittle, and subject to failure due to mechanical, physical, or chemical influences. Stress cracking is accelerated by temperature cycling, duration, temperature, chemicals, and cross-linking, orientation, or other characteristics within the plastic. Plastic optics for solar energy conversion include large-area Fresnel lens parquets commonly used in concentrator photovoltaics (CPV). Solar energy conversion takes place in a harsh environment. We look for evidence for ESC in plastics and glass optics for sunlight collection, measure stress, and discuss the possible impact of stress on longevity, optical efficiency, test methods, and manufacturing strategies.

Collimating light is a common task in optical design. The smaller the source, the simpler the optics required for collimation. Small sources cannot emit sufficient power for some demanding applications. Larger sources are required. The physical limits of collimation due to the conservation of ´etendue and skewness are discussed. Two systems are presented:a domed nonimaging Fresnel lens with toroidal primary focus and our "Umbrella" optics consisting of two free-form non-aximsymmetric mirrors.

The reflection losses, which occur during light guidance through polystyrene fibres of 30 μm diameter at their core-cladding interface, have been determined in order to evaluate their contribution to the total light attenuation of scintillating fibres. For the deviation (1 - q ) of the reflection coefficient q from unity we obtained: 6 × 10 −5 (at 458 nm), 5.36 × 10 −5 (at 514 nm) and 5 × 10 −5 (at 633 nm). In addition, we found light losses in the polystyrene core, which we attribute to Rayleigh scattering on submicron density variations in polystyrene. The resulting scattering lengths are: 3.6 m (at 420 nm) and 9.5 m (at 530 nm).

We describe optical test methods for indoor and outdoor assessment of concentrating photovoltaic (CPV) and concentrating solar power (CSP) plants and modules. Testing may be done in active mode, where the cell is driven in forward-bias mode, and passive mode, where the system or parts of it are illuminated do measure performance and detect errors. An update on the design and development of the concentrating solar simulator is given. A camera may be used to verify optical alignment and tracking sensitivity of the concentrator system. Indoor testing is required for quality assurance. It is a condition for performance guarantees and performance-based pricing.

Concentrating photovoltaics (CPV) often use Fresnel lenses, which are characterized by focal foreshortening and dispersion. These aberrations must be accounted for when designing Fresnel lenses for CPV applications. We discuss the dependence of the concentration limit of solar Fresnel lenses on dispersion in detail. For typical Fresnel lenses, dispersion creates a beam width in the order of the receiver size. Consequently, it is impossible to achieve a prescribed irradiance distribution on the target of refractive solar collectors with a concentration ratio above 500 suns, or for aspect ratios small or large.

Single-mode fiber optical receivers have become the instrumentation standard for Dynamic Light Scattering (DLS). In a regular homodyne experiment one values their superb signal-to-noise ratio as well as the simplicity of the optical setup. Moreover, mode-selective DLS enables the researcher to tackle seemingly hopeless experimental problems, such as colloidal motions inside an opaque porous medium consisting of a water filled packing of small glass grains. The particles to be measured are completely masked by strong diffuse scattering in the porous matrix. Nevertheless, mode-selective DLS makes it possible not only to detect the motions of the colloids within the pores but also to determine their diffusion coefficient and, simultaneously, their average convective speed. We outline the theoretical background of these measurements and present data on diffusion and convection of latex particles in dense packings of glass-beads in a Chromatographic column. Our technique allows an accurate determination of the tortuosity of the interstitial flow.

The Carambola is an optical device designed to allow the deterministic and multiple recycling of light rays. The rays transit through the source a defined number of times before exiting in the same phase space as light directly emitted and not recycled. The brightness enhancement by light recycling (the optical light recycling factor) with the Carambola depends on the reflectivity of the reflecting walls of the Carambola, as well as on the size of the source and on the optical thickness of the source. The results of a ray-tracing simulation and an analytical model are promising an optical light recycling factor up to three for a Xenon high-pressure arc discharge lamp.

The laws of Planck and Kirchhoff are fundamental to a physical model yielding the radiance distribution emitted from a filament light source. The filament is made of a wire coil, or a coiled coil, or even a coiled coiled coil. Some parts of the filaments face other parts. Some light is reflected or absorbed between parts of the filament. This effect is termed light recycling. Light recycling depends on the geometry of the filament, and its material properties. Our model is based on the thermodynamics, geometry, material and electrical properties of a filament lamp. Light recycling is integrated into the model. The model calculates the one-dimensional temperature distribution in the filament by solving the time dependent heat transfer equation. The results of the model are verified with absolute radiance measurements. Parameters are identified in order to increase the accuracy of the values used for material characterization. The source model may be integrated into optical software packages.

Recycling of light allows the luminance (radiance) emitted by a light source to be increased at the cost of reducing the total luminous flux (radiant power). Recycling of light means returning part of the emitted light to the source, where part of it will escape absorption. An optical design that is suitable for multiple and controlled recycling is described. Carambola optics is named for its resemblance to star fruit. Several pairs of mirrors or prisms redirect light repeatedly onto the source, thus achieving multiple transits of the light through the source. This recycled light exits the carambola in the same phase space as light directly emitted and not recycled.

The brightness theorem states that it is impossible to increase the spectral radiance of light by passive optical devices, which seems intuitive because spectral radiance is connected to temperature; increasing it seems to violate the second law of thermodynamics. However, consider a gray thermal source, that is a source that emits thermal radiation but with less than unit emissivity. Kirchhoff's law states that absorptivity is equal to emissivity. Thus if we redirect part of the emitted radiation back onto the source, some of it would not be absorbed but either transmitted or reflected instead. Consequently, this radiation would be superposed to the thermal radiation from the source in the same phase space and thereby the spectral radiance would be increased. In fact, thermal sources such as high pressure discharge lamps feature absorptivity-emissivity values far below unity. In this contribution with spectroscopic measurement we show that for such sources the spectral brightness can be increased. This does not contradict the second law because light recycling reduces the irreversible entropy production inherent to the radiation process. It is equivalent to increasing the optical thickness of sources for the price of reducing the total phase space of emitted radiation by the same factor.

The yearly energy collection efficiency of stationary and quasi-stationary (seasonally tilted) solar concentrators can be evaluated using reverse ray-tracing, and a solar radiation model. In reverse ray-tracing, rays originating at the receiver of the concentrator are traced toward the surrounding hemisphere. The resulting specular optical efficiency of the concentrator is folded with the radiance predicted by a solar radiation model yielding a data set in W/(sr m2). The method allows for the evaluation of the absolute yearly energy collection. Concentrators may be optimized for location and tilt, requiring one-time ray-tracing of the optical system, thus significantly reducing simulation time. Only the radiation model must be executed for each change of tilt. Automatically, only possible solar incidence is considered. The analysis is performed for a novel micro-structured concentrator, for the showcase location of Hawaii.

The idea of light recycling is rather simple. Assume that part of the light emitted by a light source is returned to the light source itself. If the light source does not completely absorb this light then the part which is not absorbed, is still available for further use. The hidden virtue of light recycling is that the recycled light is superposed in the same phase space (etendue) as the original radiation. Thus the average radiance of the source is increased albeit at the price of a reduction of total radiant power. This seems to violate the Second Law of Thermodynamics because the temperature of the radiation is related to the spectral radiance. Increasing the radiance amounts to reducing the entropy. However, radiating into free space is an irreversible process in which entropy is created. Light recycling reduces the entropy carried by the radiation by reducing the entropy production rate in the emission process itself. We show that the maximum radiance which can be achieved by light recycling is marked by the equilibrium radiance. The equilibrium luminescent spectrum diverges as photon energies approach the splitting of the quasi Fermi levels. The familiar spectrum of LEDs clearly does not diverge because the absorptivity/emissivity approaches zero in this regime. These features render light recycling particularly attractive. We report on preliminary laboratory measurements which show encouraging results.

A new photo—detector has been developed and operated for the first time. It is particularly suited to particle detection and tracking with scintillating fibres and to position-sensitive photon counting from low intensity light sources.

Our paper treats the conversion of light beams with circular cross-section into light beams of square cross-section as well as the conversion of light beams with rectangular cross-sections of different aspect ratios. We calculate the possible concentration ratio,and introduce symmetry-breaking microstructures in order to mix high with low skewness without affecting the axial component of the k -vector. A typical example for its application is as secondary concentrator-homogenizer in 3D-photovoltaic (PV)concentrator systems,for square PV-cells. For the performance of modern multi-junction cells it is crucial to homogenize the incoming radiation in the secondary, both in location and color. We compare the performance of homogenizers with and without microstructures and show the advantage of adding this feature. Several designs are modeled and the performance is compared by Monte-Carlo ray-tracing. With wall microstructures,light uniformity and concentration is significantly better.

Snell's law allows to find the slope of an optical surface needed to redirect a given incoming ray into a given outgoing ray. Since a prism comprises two surfaces the problem of redirecting one ray with a prism is underdetermined. In a range of situations it is possible to determine a prism such as to simultaneously match two given incoming rays into two given output rays. This allows to tailor 2D Fresnel optics for finite sources and targets. If source and target subtend equal angles as seen from the Fresnel lens, then the facets are symmetric resembling the minimum deviation configuration, which also minimizes chromatic aberration based an the dispersion in the material of the lens.

The key idea of Fresnel optics is to decouple the global slope from the local slope by breaking up the optical surface into small facets. The size of the facets is irrelevant as long as they are larger than the wavelength of light, so that the system behaves according to geometrical optics, and at the same time small compared the overall size of the optical surface. From the point of view of phase-space conservation, Fresnel optics suffer from a basic shortcoming. The phase-spaces of incoming and outgoing radiation beams need not automatically be equal. This results in either a dilution of radiation or losses or both. On the other hand, decoupling local from global slope allows to tailor the overall shape of the Fresnel lens independently from designing the individual facets. We show that it is possible to closely match incoming and outgoing radiation beams with a particular choice of the global shape of the Fresnel surface. This shape imultaneously minimizes dilution and blocking.